JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 107

Origins of in a Simulated Tornadic Mesovortex Observed during PECAN on 6 July 2015 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020 MATTHEW D. FLOURNOY School of Meteorology, University of Oklahoma, Norman, Oklahoma

MICHAEL C. CONIGLIO NOAA/National Severe Laboratory, and School of Meteorology, University of Oklahoma, Norman, Oklahoma

(Manuscript received 27 June 2018, in final form 12 October 2018)

ABSTRACT

To better understand and forecast nocturnal and their hazards, an expansive network of fixed and mobile observing systems was deployed in the summer of 2015 for the Plains Elevated at Night (PECAN) field experiment to observe low-level jets, convection initiation, bores, and mesoscale convective systems. On 5–6 July 2015, mobile radars and ground-based surface and upper-air profiling systems sampled a nocturnal, quasi-linear convective system (QLCS) over South Dakota. The QLCS produced several severe wind reports and an EF-0 . The QLCS and its environment leading up to the mesovortex that produced this tornado were well observed by the PECAN observing network. In this study, observations from radiosondes, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on the development of the observed tornadic mesovortex. All ensemble members simulated low-level with one member in particular generating two mesovortices in a manner very similar to that observed. Forecasts from this member were analyzed to examine the processes increasing vertical vorticity during the development of the tornadic mesovortex. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized downdraft/rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing mesovortex.

1. Introduction convectively induced, midtropospheric vortices, including bookend vortices (e.g., Weisman 1993) and mesoscale Mechanisms responsible for the development of sig- convective vortices (MCVs; e.g., Menard and Fritsch 1989; nificant low-level1 rotation and mesovortices in quasi- Cotton et al. 1989), which are typically 20–200 km in length linear convective systems (QLCSs) have been studied and can persist for several hours. Mesovortices are typi- for decades. Mesovortices are typically meso-g-scale cally most intense near the surface (1–2 km AGL), thus (Orlanski 1975; 2–20 km) phenomena, extend up to 3 km explaining their association with straight-line surface wind above ground level (AGL), and persist for no more than an damage and, occasionally, tornadoes. This inherent severe hour. These features distinguish mesovortices from other weather threat associated with mesovortices has made them the subject of many studies, which have yielded multiple theories as to how they form and produce strong 1 In this paper, the term ‘‘low level’’ is used to describe features winds and tornadoes. in the 500–2000-m AGL layer. a. Mesovortices in QLCSs

Corresponding author: Matthew Flournoy, matthew.flournoy@ Numerous observational studies have shown that noaa.gov mesovortices are associated with severe surface winds

DOI: 10.1175/MWR-D-18-0221.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 108 MONTHLY WEATHER REVIEW VOLUME 147 within QLCSs (e.g., Forbes and Wakimoto 1983; Funk et al. 1999; Przybylinski et al. 2000; Atkins et al. 2004, 2005; Wakimoto et al. 2006b; Schenkman et al. 2011a; Mahale et al. 2012; Newman and Heinselman 2012). Such mesovortex-induced wind events tend to be localized in space and time, on the order of tens of kilometers and minutes, and can develop very quickly within QLCSs (Mahale et al. 2012; Newman and Heinselman 2012). Because of this, forecasting severe wind associated with mesovortices is a difficult problem, Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020 and the study of both (i) synoptic conditions conducive for the development of mesovortices and (ii) mecha- nisms forcing mesovortex genesis in QLCSs remain ac- tive areas of research. 2 Given favorable thermodynamic conditions, 15–20 m s 1 FIG. 1. Schematic showing the formation of a cyclonic MV via of line-normal vertical in the lowest 5 km of the the ‘‘ mechanism’’ by Atkins and St. Laurent (2009b; e.g., environment is conducive for the development of meso- tilting and stretching of low-level horizontal and vertical vorticity vortices (Weisman and Trapp 2003). These values are transported to the surface by a downdraft). Blue and red arrows qualitatively consistent with Rotunno–Klemp–Weisman represent -internal and environmental airflow, respectively, (RKW) theory (Rotunno et al. 1988), which explains how and gold arrows represent low-level horizontal vortex lines. The green arrows show the location of the cyclonic MV. strong cold pool–forced updrafts result from a balance between cold pool and low-level environmental vertical wind shear oriented normal to the convective line. originated from upward tilting of environmental hori- Whereas a stronger cold pool–forced updraft plays a role in zontal vorticity (Wheatley and Trapp 2008). This has also the structure and intensity of the convective line, a stronger been shown theoretically by Lee and Wilhelmson (1997a,b), forced updraft could also favor mesovortices through in- who found misovortices (and tornado-like vortices) to creased vertical vorticity stretching and intensification. form along a dry outflow boundary via vortex sheet Quasi-idealized simulations of observed mesovortices have rollup, subharmonic interactions, and consolidation of yielded similar results, with stronger mesovortices forming dominant vortices. Although this mechanism may be when low-level environmental wind shear appeared to important for the genesis of some QLCS mesovortices, sufficiently balance cold pool shear (Atkins and St. Laurent many have been observed to form in isolation, in 2009a). These results agree with the observational study of cyclonic–anticyclonic couplets, or at spacings along the Schaumann and Przybylinski (2012), who found that in- gust front not predicted by a horizontal shear instability tense mesovortices capable of producing tornadoes are mechanism, suggesting that other processes must be at more likely when wind shear is oriented normal to the least partially responsible (e.g., Trapp and Weisman convective line in the lowest 3 km. In other modeling 2003; Atkins and St. Laurent 2009b; Schenkman et al. studies, Snook et al. (2011) and Schenkman et al. (2011b) 2012; Xu et al. 2015). found that an accurate representation of low-level envi- One of these mechanisms for mesovortex formation in ronmental shear was crucial for capturing the location and QLCSs is the tilting of baroclinic vorticity generated intensity of mesovortices. Although mesovortex generation internally along the cold pool interface by an updraft or is strongly associated with low-level shear, mesoscale het- downdraft. This process generates a couplet of positive erogeneities (Wheatley and Trapp 2008) and storm- and negative vertical vorticity along the gust front, with generated features (Newman and Heinselman 2012)can the orientation of the couplet dependent upon whether also play a role. an updraft (Atkins and St. Laurent 2009b) or downdraft One of the first studies investigating QLCS mesovortex (Bernardet and Cotton 1998; Trapp and Weisman 2003; formation on the mesoscale hypothesized that shear in- Wheatley and Trapp 2008; Richter et al. 2014) serves as stability played a primary role (Forbes and Wakimoto the tilting agent. 1983). Some later studies came to the same conclusion A third mechanism for mesovortex formation in (Przybylinski 1995; Wheatley and Trapp 2008), showing QLCSs (specifically for isolated, cyclonic mesovortices) that the release of horizontal shear instability (Miles and is the tilting and stretching of low-level, predominantly Howard 1964) along a system-generated gust front served streamwise horizontal vorticity by an updraft. This as the precursor to mesovortex development. The vertical process differs from that responsible for the formation vorticity relied upon for the shear instability mechanism of mesovortex couplets because the updraft becomes JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 109 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 2. The 500-hPa geopotential heights (black lines every 60 m), temperature (dashed red lines every 28C), and winds (full barb 5 10 kt) from the RAP analysis valid at 0000 UTC 6 Jul 2015 (from the Storm Prediction Center archive). collocated with the vertical vorticity maximum, study, Xu et al. (2015) found a large residual horizontal producing a single rotating updraft (Fig. 1). This low- vorticity tendency after accounting for the effects of level vertical vorticity is typically generated by tilting of tilting, stretching, and baroclinicity along parcel trajec- horizontal vorticity in association with a downdraft (e.g., tories; the study presumed that the residual vorticity Atkins and St. Laurent 2009b; Parker and Dahl 2015) tendency was largely the result of frictional effects, as along an internal boundary (i.e., the system gust front) opposed to errors in the other budget terms. The authors or an external boundary with preexisting vertical vor- concluded that frictionally generated vorticity contrib- ticity (Przybylinski et al. 2000; Schenkman et al. 2011b) uted significantly to the development of a mesovortex prior to encountering the low-level updraft. These in a QLCS. Frictional effects have also recently been mechanisms and the resulting collocation of the updraft shown to serve as an important source of vorticity in and cyclonic vorticity resemble the processes responsi- supercellular and tornadoes in modeling ble for the formation of low- and midlevel mesocyclones studies (Schenkman et al. 2014; Markowski 2016; Mashiko and tornadoes in (e.g., Markowski and 2016b; Roberts et al. 2016); however, Markowski and Richardson 2009). Bryan (2016) found that the use of large- simula- Recent studies add that horizontal vorticity generated tions may result in the development of unrealistically by surface friction may be an important source for large near-surface shear in unstratified, horizontally mesovortex generation. In real-data simulations of a homogeneous, quasi-steady boundary layers if the flow nocturnal QLCS, Schenkman et al. (2012) concluded is not sufficiently turbulent. Thus, the role of surface that both surface friction and boundary layer stability friction as an important vorticity source in convective played a role in the development of an intense, low-level environments remains a relatively unexplored, active ‘‘rotor’’ of horizontal vorticity just behind the gust front. area of research. The upward branch of this rotor coincided with an in- Many of the above processes responsible for the tense mesovortex and tornado-like vortex. In a similar generation of mesovortices can be augmented by the 110 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG.3.AsinFig. 2, but for 2-m temperature (red and magenta contours every 58F), 2-m dewpoint temperature (blue dashed contours shaded every 48F), and pressure reduced to mean sea level (black contours every 4 hPa). presence of a rear-inflow jet (RIJ), a relatively common b. Tornadoes in QLCSs feature of mature QLCSs (Weisman 1992) that is fre- While nearly all violent (EF4–5) tornadoes are spawned quently associated with severe winds at the surface. by supercells, it is estimated that 18% of all tornadoes are Severe winds may or may not be associated with low- associated with QLCSs (Trapp et al. 2005). Given the level mesovortices, but when they are, the strongest propensity of QLCSs to persist overnight, these torna- winds are typically found within the equatorward por- does are more likely to occur in the late night and early tion of the mesovortex (Atkins and St. Laurent 2009a), morning hours than supercellular tornadoes (Trapp et al. where the motion of the system, enhanced winds in the 2005). This characteristic, in combination with lead times RIJ, and internal circulation of the cyclonic mesovortex limited to an average of 5 min (Trapp et al. 1999), can are all oriented in the same direction (Wakimoto et al. render QLCS tornadoes just as dangerous to the public as 2006a). RIJs often descend toward the leading edge of their supercellular counterparts (Ashley 2007; Ashley the QLCS, which can enhance convergence at the gust et al. 2008). Thus, the investigation of low-level processes front and subsequent low-level updrafts, vorticity tilting, influencing in QLCSs remains an impor- and stretching, and result in more intense mesovortices tant area of research. (Atkins et al. 2005; Schaumann and Przybylinski 2012; As in the case of supercellular tornadoes, QLCS tor- Newman and Heinselman 2012; Xu et al. 2015). Strength- nadogenesis appears to be directly related to the presence ening mesovortices above the ground can lead to an of a low-level mesovortex. Atkins et al. (2004, 2005) upward-directed vertical pressure perturbation gradient found all tornadoes in two damaging Midwest to force, which in turn enhances low-level updrafts and be associated with low-level mesovortices. These meso- strengthens the initial mesovortices. This mechanism vortices were stronger, deeper, and lasted twice as long as may be responsible for the intensification of meso- nontornadic mesovortices; this is an intriguing finding, as vortices that are located near an RIJ (e.g., Xu et al. 2015) supercellular mesocyclones that produce tornadoes may and, in some cases, the potential for a mesovortex to not differ significantly from those that do not (Atkins produce a tornado (Atkins et al. 2005). et al. 2004). Atkins et al. (2005) also concluded that the JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 111 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 4. Skew T–logp diagram and 0–6-km hodograph (kt in upper left) from a radiosonde launched at 0325 UTC 6 Jul 2015 at the location of the star in Fig. 5. The dashed red line is the virtual temperature, and the black dashed line is the virtual temperature of the most unstable parcel. Variables computed from three different parcels are shown in the upper right.

2 stronger mesovortices associated with tornadoes may (1 kt ’ 0.5144 m s 1) jet moved through Manitoba and have been forced by a descending RIJ (e.g., Schaumann Ontario (Fig. 2). Northwesterly, 500-hPa flow of 15– and Przybylinski 2012; Xu et al. 2015). 20 kt over southeastern South Dakota transitioned to The present study provides an examination of the 20–30-kt westerly flow during the next 6 h (not shown) processes leading to mesovortex genesis and tornado- as a trough moved eastward into the Dakotas. A surface genesis in a QLCS that occurred on 6 July 2015 during front stretched from western South Dakota to a broad the Plains Elevated Convection at Night (PECAN) field region of low pressure east of the Front Range of the experiment. The next section features an overview of Rockies (Fig. 3). The southerly and southeasterly flow the synoptic conditions during the event as well as the east of this low-pressure area advected warm, moist air PECAN assets that were deployed that night, followed northward into South Dakota with temperatures near- by a description of the configuration used for the simu- ing 908F and dewpoints above 708F. Even though aver- 2 lation of the mesovortex. Simulation results related to age midlevel lapse rates were only 68–78Ckm 1 (not mesovortex development are described in section 4, shown), this warm, moist boundary layer supported followed by conclusions and discussions in section 5. most unstable convective available potential energy 2 values above 5000 J kg 1 (see Fig. 4). Although the 2. The 6 July 2015 PECAN case deep-layer vertical wind shear was relatively weak (20– 30 kt over the lowest 4–6 km AGL; Fig. 4), it was suffi- a. Synoptic overview cient to support loosely organized multicell convection At 0000 UTC 6 July 2015, weak midlevel westerly capable of damaging wind gusts and hail. However, low- flow was present over the Rockies while a 50–60-kt level winds strengthened, and storm-relative helicity 112 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 5. Hourly composite of 40-dBZ reflectivity contours from 0000 to 0600 UTC 6 Jul 2015 from the NSSL Multi-Radar Multi-Sensor System. The final locations of eight mobile radars and baselines between them (30–50 km) are shown in gray. The star represents the location from which a sounding was launched from at 0325 UTC (see Fig. 8), and the gold box indicates the zoomed-in view of Fig. 6.

increased in the lowest 1.5 km AGL (Fig. 4) prior to the systems while releasing radiosondes every hour. Three arrival of convection in the PECAN domain (Fig. 5), aircraft collected both in situ and microphysical data supporting more robust, rotating updrafts and a risk for during the deployment ahead of the convective lines and the development of low-level mesovortices and associ- along spiral ascents/descents within the trailing strati- ated tornadoes. form precipitation. Other fixed observing sites across Kansas and Nebraska released radiosondes at 3-h in- b. PECAN field deployment tervals. A subset of these data was assimilated into an The field phase of PECAN occurred from 1 June to ensemble analysis and forecasting system (described in 15 July 2015 with the goal of furthering our understanding section 3) including in situ data from two aircraft, data of continental, nocturnal, warm-season precipitation. This from six mobile radars, three WSR-88Ds, and radio- overarching goal included specific foci on four atmospheric sondes. Interested readers are referred to Geerts et al. phenomena that included mesoscale convective systems (2017) for details of these PECAN observing systems. (MCSs). During the evening of 5 July, PECAN assets were deployed ahead of an MCS that developed over south- 3. Numerical simulation configuration central South Dakota around 0000 UTC 6 July (Bodine and Rasmussen 2017) and moved through the PECAN A predecessor system that has since evolved into the observing network over southeast South Dakota (Fig. 5). NSSL Experimental Warn-on-Forecast (WoF) System A pair of cyclonic mesovortices formed over the northeast for ensembles (NEWS-e; Wheatley et al. 2015; Jones et al. portion of the network (Fig. 6), with the northern meso- 2016; Lawson et al. 2018) was used for simulating the vortex becoming much stronger and producing an EF-0 5–6 July 2015 MCS. This system assimilates observations tornado. using an ensemble Kalman filter (EnKF; Evensen 1994) Three collocated mobile radiosonde systems stag- into an ensemble of model states created by WRF-ARW gered the release of radiosondes such that vertical pro- version 3.6.1 (Skamarock and Klemp 2008) using the files were obtained approximately every 20 min as the Data Assimilation Research Testbed (Anderson et al. MCS approached (their location is indicated by the star 2009). The ensemble has 36 members that are created by in Fig. 5). Separate mobile profiling systems were de- using different parameterization schemes for longwave ployed at other locations in the network and collected and shortwave radiation, the surface layer for heat, continuous data from a variety of remote sensing moisture, and momentum fluxes, and turbulent mixing in JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 113 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 6. Schematic showing the paths of the two observed low-level mesovortices, EF-0 tornado, and NOXP. The observed mesovortices locations were determined using mobile and WSR-88D radar observations from Sioux Falls, SD.

the planetary boundary layer (PBL), as well as by using 24 data assimilation cycles from 0000 to 2300 UTC 5 July the initial conditions provided by 18 members of the (Fig. 8). These hourly assimilation cycles included con- Global Ensemble Forecast System [GEFS; the configu- ventional observations from the Meteorological Assimi- ration is the same as that detailed by Hitchcock et al. lation Data Ingest System (MADIS; Miller et al. 2007) (2016)]. Please reference Wheatley et al. (2015) for a full and PECAN radiosondes. The MADIS observations in- description of the physics options used for the first 18 clude (i) mandatory and significant levels from NWS ra- GEFS members (see their Table 2). For members 19–36, diosondes; (ii) surface data from routine aviation weather these physics options are applied in reverse order (i.e., member 19 is constructed by applying physics option 1 to GEFS member 18, etc.) such that no member is identi- cally configured. The Thompson microphysics scheme including six classes of moisture species and ice number concentration as prognostic variables was used in all members (Thompson et al. 2004); the microphysics scheme was not perturbed between members. A total of 51 vertical levels were used, with the spacing varying from 25 m near the surface to near 780 m at the model top (50 hPa). Eight vertical levels were located in the lowest 1kmAGL. A series of four nested grids was used, with the three inner grids nested one way with their outer grid (Fig. 7). Horizontal grid spacings were 15 km, 3 km, 1 km, and 333 m. The ensemble state was initialized by interpolating FIG. 7. Domain configuration for the simulations. The domains the GEFS analyses valid 1800 UTC 4 July to the 15-km are labeled by their horizontal grid spacings (15 km, 3 km, 1 km, grid and was integrated in WRF-ARW for 6 h before and 333 m). 114 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 8. Synthesis of the data assimilation for this ensemble simulation. Analyses and forecasts produced on each grid are shown above the black timeline, and time periods in which the data from each platform were used in the 15-min assimilation windows are shown below the black timeline. The three WSR-88D radars included Sioux Falls (KFSD), North Platte (KLNX), and Aberdeen (KABR). The six mobile radars included three Doppler on Wheels, two Shared Mobile Atmospheric Research and Teaching Radars, and NOAA X-band Polarimetric Radar. reports (METARs), marine reports (from both ships and observations were only assimilated if reflectivity . 10 dBZ. buoys), the Oklahoma , and mesonet observa- The radiosonde observations were quality controlled tions from a variety of national networks; (iii) Aircraft and thinned to 50–60 vertical levels prior to assimilation. Meteorological Data Relay (AMDAR) reports for wind At 0300 UTC, the analysis on the 3-km grid was used to and temperature; and (iv) atmospheric motion vectors initialize the 1-km grid, and 1-km forecasts were gen- derived from satellite observations (as in Coniglio erated out to 0600 UTC (using 3-km analyses for the et al. 2016). At 2300 UTC 5 July, the 3-km grid was initialized from the 15-km analysis and was integrated for 1 h. Assimilation on the 3-km grid was performed from 0000 to 0600 UTC 6 July every 15 min and included ra- diosonde observations, Doppler radial velocity (e.g., Snyder and Zhang 2003; Zhang et al. 2004), and re- flectivity (e.g., Dowell et al. 2004; Tong and Xue 2005; Aksoy et al. 2009, 2010; Yussouf and Stensrud 2010; Dowell et al. 2011; Yussouf et al. 2013) from the mobile and WSR-88D radars listed in Fig. 8. All radar data were quality controlled and dealiased using Py-ART software (Helmus and Collis 2016) as well as subjectively using the Earth Observing Laboratory Solo3 software. The reflectivity and Doppler velocity data were then ana- lyzed onto a 6-km grid with a two-pass Barnes scheme (Barnes 1964) using Observation Processing and Wind Synthesis (OPAWS) software before assimilation onto 2 the 3-km grid. Standard errors of 5 dBZ and 2 m s 1 were FIG. 9. Vertical vorticity swaths averaged in the lowest four model levels (approximately the lowest 150 m AGL) exceeding assumed for the reflectivity and velocity observations, 2 0.01 s 1 from 0400 to 0600 UTC (1–3-h forecasts) at 5-min intervals respectively, during the EnKF assimilation process, for each ensemble member (indicated by the different colors). similar to many studies that assimilate Doppler radar Observed mesovortices and the tornado track from Fig. 6 are observations (e.g., Yussouf et al. 2013). Radial velocity overlaid. JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 115 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 10. Simulated lowest model-level reflectivity (scale in lower left), vertical vorticity (pink contours with scale in lower right), and ground-relative winds (scale in lower right) from member 30 from 0435 to 0451 UTC on the 333-m grid. (a) Black star represents the location that the simulated hodographs were extracted from in Fig. 13. lateral boundary conditions). The 1-km forecasts valid 4. Simulation results at 0345 UTC from several ensemble members were then a. Comparison to observations used as the initial and boundary conditions for forecasts on the 333-m grid at 0345 UTC.2 Forecasts for these All of the ensemble forecasts produce an MCS in their members were created through 0600 UTC 6 July on the forecasts with a general evolution similar to the observed 333-m grid. evolution of the system. In addition to simulating the general structure of the MCS, many of the 1-km forecasts produce mesovortices of varying intensity and longevity 2 Tests were performed with data assimilated onto the 1-km grid (Fig. 9). When compared to the observed mesovortex every 5 min in an attempt to constrain the analyses on the 1-km grid tracks (Fig. 9), the ensemble forecasts contain significant further toward observations. However, forecasts generated from low-level rotation about 20–30 km west of the observed these 1-km analyses were inferior to those generated from the tornadic and nontornadic mesovortices. Inspection of the downscaled 3-km analyses, with too much spurious convection ensemble forecasts’ evolution of the MCS shows that this generated shortly after the forecasts were initialized (not shown). This is an ongoing research problem within storm-scale assimila- is the result of the system evolving faster than in nature. tion systems on grids O(1) km (D. Wheatley 2016, personal However, careful analysis of storm-scale features re- communication). veals many similarities between the 333-m forecasts and 116 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 11. As in Fig. 10, but for lowest model-level density potential temperature (ur), perturbations (shaded), and storm-relative winds. (i) The MV that is the subject of this study is labeled as MV1. observations in the development of the mesovortices. ahead of the main east-southeastward-moving MCS This is especially true for member 30, as detailed below. (Figs. 10 and 11). Convection was similarly observed to Parameterizations for this specific member included the develop ahead of the main system (Figs. 12a,c). The Rapid Radiative Transfer Model for general circulation forecasts also contain a west-southwest–east-northeast- models (RRTMG) longwave and shortwave radiation oriented band of convection to the north and east of the schemes, RUC surface physics, MYNN surface-layer main MCS, seen moving to the south from the upper physics, and a 3D TKE turbulent diffusion scheme (no part of the panels in Fig. 10. A west-southwest–east- planetary boundary layer scheme was used). northeast-oriented band of convection with a well- A complex storm-scale evolution is evident in mem- defined gust front also was observed (Fig. 12a) that ber 30 forecasts valid from 0435 to 0455 UTC (Fig. 10). moved steadily to the south and east (Fig. 12c). Outflow generated by convective downdrafts (e.g., over Southeasterly mesovortex-relative near-surface winds 2 the lower-right portion of the panels in Fig. 10) is evident of 15–20 m s 1 were present ahead of the gust front JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 117 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

21 FIG. 12. (a),(c),(e) Reflectivity (dBZ) and (b),(d),(f) radial velocity (m s ) observations from SR1 (location shown in uppermost left panel) at a scanning elevation of 0.58 at 0415, 0445, and 0515 UTC. Approximate locations of the two gust fronts described in the text are indicated by the blue lines. The tornadic and nontornadic mesovortices at 0515 UTC are labeled and circled in the lowermost right panel. The horizontal scale is shown in (d). 118 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 13. Evolution of both observed and simulated low-level hodographs every 12 min from 76 to 16 min prior to the time the MV passed by the Collaborative Lower-Atmospheric Mobile Profiling System (CLAMPS) for the ‘‘observed’’ hodographs (0510 UTC) or the time of maximum near-surface (i.e., lowest model level) vertical vorticity associated with the mesovortices for the ‘‘simulated’’ hodographs (0447 UTC). (a)–(f) Observed hodographs plotted from 0354 to 0454 UTC and retrieved from a Doppler JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 119

(Fig. 11) because of a combination of strong larger-scale radial velocity observations available every 5 min from ground-relative near-surface winds (Fig. 10) combined Sioux Falls, South Dakota (KFSD), suggest that two 2 with the motion of the system of approximately 12.5 m s 1 separate mesovortices may have merged to form the at 2908. The outflow ahead of the main MCS did little to single tornadic mesovortex observed at 0525 UTC modify the near-surface thermodynamic state (Fig. 11) (Fig. 15). While 5-min intervals from KFSD are not fre- but did enhance the near-surface wind speeds and quent enough to deduce mesovortex evolution/mergers backed the winds to a more southerly direction. This and make a clear determination of which mesovortex added to the already substantial low-level vertical wind observed at 0520 UTC (Fig. 15d) becomes the torna- shear in the simulated environment that was oriented dic mesovortex observed at 0525 UTC (Fig. 15f), it nearly parallel to the gust front (Fig. 13), which is appears that the more northern of the two meso- Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020 consistent with observations from velocity–azimuth vortices at 0520 UTC intensifies rapidly and ingests display wind retrievals from a Doppler wind lidar ob- the southern mesovortex by 0525 UTC. This is quali- tained in the region ahead of the main system in the tatively very similar to what occurs in the simulation enhanced southerly winds. (Fig. 14), as the more northern of the two observed The observed tornadic mesovortex (and the strongest mesovortices forms near the west-southwest–east- mesovortex in the forecasts, labeled as MV1 in Fig. 11i) northeast-oriented gust front, and the southern ob- both occur near (within 20 km of) the interface of the served mesovortex forms along the north side of the gust fronts associated with the main MCS and the west- leading edge of the strong rear-inflow surge (Figs. 15a,b). southwest–east-northeast-oriented band, as well as in Although the simulated near-surface winds do not attain the vicinity of the convection that developed ahead tornadic intensity in the vicinity of MV1 (e.g., Fig. 10), of the main MCS. A zoomed-in view of the evolution the storm-scale similarities of the observed mesovortex of MV1 in the simulation is shown in Fig. 14. What to the simulated mesovortex near the location of the developed into a single mesovortex by ;0449 UTC merging gust fronts and along a rear-inflow surge (Fig. 11i) formed from a merger of two smaller meso- justify examining the simulation further to deduce the vortices (labeled MV1a and MV1b in Fig. 14c). The processes involved in the development of the tornadic cyclonic vertical vorticity within MV1a develops along mesovortex. the western edge of the west-southwest–east-northeast b. Analysis of the simulated mesovortex gust front and slowly strengthens as it nears the main MCS gust front. MV1b develops on the gust front of the To analyze the evolution of MV1 (Fig. 11i), air parcel main MCS and is associated with the strong localized trajectories [following an iterative method described by rear-inflow surge located just to its west (Fig. 10). It is Seibert (1993)] were initialized at 0435 UTC in a volume also important to note the development of a persistent centered on the path of the developing mesovortex and just to the west of MV1a (Figs. 14c–g). integrated forward in time for 20 min. Parcels were ini- While there was not clear evidence from any of the tially spaced every 500 m over a 50 km 3 50 km area, PECAN mobile radars that the observed tornadic mes- with parcels every 100 m in the vertical from 100 to ovortex resulted from a merger of two mesovortices,3 2500 m AGL, yielding 255 025 parcels total. Model output every 5 s was used to compute the trajectories as well as vertical vorticity, vertical vorticity tendency 3 The mobile X-band radars were attenuated by the abundant terms, and other meteorological quantities along the convective precipitation, and the mesovortex was at the edge of the trajectories. Parcels that contributed to the develop- range of the C-band SR1 and SR2 radars at this time. ment of MV1 at any given time were defined using a few

wind lidar on the CLAMPS (see Geerts et al. 2017). The profiles are smoothed lightly in time and height using a Gaussian filter to remove small-scale noise. The DWL profiles begin at 113 m AGL because of limitations of the lidar in sampling the near field and because of the light smoothing applied. However, the observed wind profile is extrapolated down to approximately 2 m AGL (red dashed line) using the wind observation from the mobile mesonet closest to CLAMPS at the specified time (red circle). The MCS gust front passed the CLAMPS location at about 0510 UTC, 16 min after the hodograph shown in the last panel. Simulated hodographs were extracted at the location indicated in Fig. 10a every 12 min from 0355 to 0431 UTC. Simulated hodographs are not shown in (a),(b) because forecasts on the 333-m grid began at 0345 UTC. Heights AGL (km) are indicated along each hodograph, and the ‘‘3’’ in the last panel rep- resents the observed MV motion. 120 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 14. Zoomed-in view following the evolution of what becomes MV1 (see Fig. 11i) with 1-km AGL vertical 2 vorticity (pink contours), 500-m horizontal vorticity (arrows), and 500-m AGL vertical velocity .0.5 m s 1 2 (,20.5 m s 1; green/brown contours). Positive vertical vorticity contours are solid, and negative vertical vorticity contours are dashed. The two mesovortices that merge to form MV1 are labeled as MV1a and MV1b in (c). Valid times in UTC are indicated in the lower right of each panel. The 4-km-wide search box used at 0447 UTC is shown, but for the sake of clarity is not shown at other times. constraints. A 4-km-wide search box (Fig. 14g) centered Using the above method, approximately 60–80 parcels on MV1a (labeled in Fig. 14c) over time was used, as were found to be in the mesovortex at any time. The 2 well as a vertical velocity threshold of 1 m s 1, a verti- median, 90th percentile, and 10th percentile vertical 2 cal vorticity threshold of 0.006 s 1 (i.e., the first pink velocities and vertical vorticities of parcels located contour in Figs. 10 and 11), and a height threshold of within the mesovortex at each time step are shown in 250–2000 m AGL. The box was centered on MV1a since Fig. 16. The median vertical velocity of the parcels this mesovortex intensified more rapidly than MV1b in the within the mesovortex rapidly increased from 0435 time period examined and was associated with a stronger to 0440 UTC along with a gradual increase in median updraft throughout the intensification period (Fig. 14). vertical vorticity. For the next 10 min, the median JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 121 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

21 FIG. 15. Radial velocity observations (m s ; scale at the bottom of each panel) from KFSD (relative direction shown in the upper-left panel) approximately every 5 min from 0506 to 0530 UTC at approximately 500 m AGL. The rear-inflow surge discussed in the text is highlighted in the upper-left panel. The two mesovortices that merge to form the single, observed, tornadic mesovortices are circled in pink and yellow. 122 MONTHLY WEATHER REVIEW VOLUME 147

vertical velocity and vorticity both decreased over the next few minutes. Parcels that are in the mesovortex from 0441 to 0447 UTC originated from multiple source regions in- cluding the environment and storm-cooled air behind the gust fronts. Parcels originating in the environment (referred to as the environmental source region) resided close to the surface (100–300 m AGL) and had little vertical vorticity before reaching the gust front and low-

level mesovortex updraft (Fig. 17). These parcels rose Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020 within the forced updraft along the gust front as they flowed poleward and were then pushed eastward by theleadingedgeofthemainMCScoldpool.These parcels gradually attained higher vertical vorticity values during this period (shown by the thickening lines in Fig. 17) before rising more rapidly upon en- countering the low-level updraft associated with the mesovortex. This process occurred throughout the entire intensification stage of the mesovortex. A sec- ond source region behind the west-southwest–east- northeast-oriented gust front is evident north of the mesovortex (henceforth deemed the northern source region). Parcels from this source region originated at low levels and gradually attained more vertical vor- ticity and ascended as they approached the developing mesovortex (Fig. 18). The parcels then rose rapidly upon encountering the mesovortex low-level updraft. The third source region originated well to the north- west of the developing mesovortex and was associated with a localized downdraft/rear-inflow jet (henceforth deemed the RIJ source region). These parcels origi- nated near 1 km AGL with larger initial cyclonic vertical vorticity than the parcels in the other source regions but descended before rising rapidly in the low-level mesovortex updraft (Fig. 19). The development of vertical vorticity in the parcels

FIG. 16. Median, 10th, and 90th percentiles of (top) vertical from the environmental source region is examined fur- vorticity and (bottom) updraft speed for parcels located in the ther with composite time series of the streamwise and mesovortices from 0435 to 0455 UTC. The vertical vorticity and crosswise horizontal vorticity components (defined with updraft thresholds used to determine if a parcel was located in the respect to the storm-relative wind), along with the ver- mesovortices at a given time are marked with a dashed, gray line in each plot. In total, 60–80 parcels were located in the mesovortices tical vorticity tilting and stretching tendencies (Fig. 20). at any given time from 0435 to 0455 UTC. Integrated vertical vorticity along these environmental trajectories is very similar to interpolated vertical vor- ticity (Fig. 21), so that analyzing the evolution of vertical vertical vorticity in the mesovortex continued to grad- vorticity using the tendency terms is appropriate (the ually increase while the median vertical velocity fluctu- same is not true for the other source regions). The 2 ated around 5 m s 1. The median vertical velocity and ‘‘composite’’ tilting and stretching terms were found by vertical vorticity were strongest from 0447 to 0449 UTC, first calculating tilting and stretching for each individual 2 with a median updraft speed near 5 m s 1 and vertical parcel and then calculating the 10th, 50th, and 90th 2 vorticity approaching 0.02 s 1. This occurred at the percentiles (as shown in Fig. 20). The parcel time series same time as vertical vorticity values at the surface were then shifted in time such that the time of maximum peaked and as the pair of mesovortices merged stretching tendency (i.e., as the parcel encounters the (Fig. 14). After 0450 UTC, the mesovortex decayed as low-level updraft) along each trajectory occurs at the JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 123 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 17. Trajectory traces for an area zoomed in over the environmental parcels prior to their ingestion into the mesovortices updraft at 0447 UTC. The heights AGL of the parcels are indicated by their color (scale on the right), and the vertical vorticity of the parcels is indicated by their thickness (scale in the upper right of each panel). Red (black) arrows show the storm-relative wind (horizontal vorticity) at the parcel location at the time (UTC) in- dicated in the lower left of each panel (scales for the wind and horizontal vorticity are given in the lower right). Lowest model-level vertical vorticity is contoured in pink. same time (defined as t 2 0inFig. 20). The tilting and decreases and even becomes negative for some parcels stretching terms appear rather intuitive as the parcels prior to encountering the low-level mesovortex updraft. encounter the low-level mesovortex updraft, with the Figure 20 shows that upward tilting of horizontal tilting term gradually increasing prior to a rapid increase vorticity is the primary contributor to the initial gener- in stretching and a subsequent gradual decrease in tilting ation of cyclonic vertical vorticity in the environmental as the horizontal vorticity is converted to cyclonic ver- parcels at low levels (2–4 min prior to their ingestion into tical vorticity. However, it is notable that the horizontal the MV), followed by large stretching of vertical vor- vorticity associated with these parcels was mostly ticity as the parcels enter the eastern periphery of the crosswise prior to and during tilting, at least for the low-level updraft (0–2 min prior to their ingestion into parcels located within the mesovortex at 0443–0447 the MV). Figure 17 shows this can be visualized as UTC (Figs. 20b–d). This large crosswise vorticity is northward-flowing parcels with horizontal vorticity related to the large near-ground vertical wind shear pointing westward that are tipped upward within the oriented at a small angle to the gust front and storm- gust front updraft, resulting in cyclonic vertical vorticity. relative flow (Fig. 13). For parcels located within the This process occurs prior to the parcels entering the mesovortex at 0445 and 0447 UTC, streamwise vorticity updraft of the developing mesovortex. During this time, 124 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 18. As in Fig. 17, but for the parcels from the northern airstream. the horizontal vorticity vectors of these inflowing parcels contributes to more rapid tilting of the horizontal vor- turn somewhat toward the southwest, especially for the ticity than would otherwise occur without the surge. This parcels with largest vertical vorticity values closest to suggests that a role of the rear-inflow surge in this case is the mesovortex (Fig. 17d). This change in orientation of to force the environmental inflow upward more rapidly the horizontal vorticity vectors to the southwest is likely on the equatorward side of the surge, with the geometry related to the baroclinic generation of horizontal vor- of the bowing gust front such that the parcels emerge near ticity along the gust front, which generates a component the developing mesovortex on the poleward side of the of horizontal vorticity generally pointing southward (for bowing gust front with significant cyclonic vertical vor- the eastward-pointing density potential temperature ticity. Furthermore, the trajectories have a small angle gradient along the north–south-oriented gust front). relative to the system-wide gust front, ensuring that the However, the orientation of the vorticity vectors is not parcels will reside in the region of forced upward accel- altered enough to eliminate the substantial crosswise eration longer than they would otherwise be with a more vorticity inherent in the strong low-level shear prior to westerly trajectory to the inflow parcels that is typically the parcels reaching the developing mesovortex. seen in past mesovortex simulations (e.g., Trapp and Finally, for the environmental parcels, it is notable that Weisman 2003; Atkins and St. Laurent 2009b). In this the strong surge of horizontal winds within the localized configuration of inflowing parcels and the gust front, downdraft/rear-inflow jet that impinges on the strong without the surge and the protruding bow in the gust front southerly low-level flow enhances the convergence and (say for parcels farther south), the vorticity vectors are forced updraft along the gust front (Fig. 14), which still tilted upward along the gust front updraft, but this JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 125 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 19. As in Fig. 17, but for the parcels from the RIJ airstream. upward tilting does not occur as quickly, and the parcels the low-level mesovortex updraft, parcels rise very little are well behind the gust front in or near the top of the cold and are characterized by both streamwise and crosswise pool by the time they acquire cyclonic vertical vorticity in horizontal vorticity. However, from about 2 to 6 min this way. prior to encountering the low-level mesovortex updraft, The same analysis was performed for parcels origi- horizontal vorticity increases and becomes much more nating in the northern source region (Fig. 22), with one streamwise. This is evident in Fig. 18, as initially chaotic, caveat. Since the vertical vorticity derived from the in- small horizontal vorticity (Figs. 18a,b) becomes oriented tegrated budget terms was close to the interpolated toward the south (Fig. 18c) and southwest (Fig. 18d)in vertical vorticity for only a small number of parcels, the direction of the storm-relative wind as the parcels those budget terms were not included in this analysis. near the low-level updraft. This illustrates that these Rather, a more qualitative analysis is performed for parcels are providing a source of primarily streamwise these parcels with updraft and mean height AGL vorticity from low levels directly into the developing (Fig. 22). Time series are rather short for parcels located mesovortex. The streamwise vorticity develops as the in the mesovortex at 0441, 0443, and 0445 UTC parcels pass though the baroclinic zone associated with a (Figs. 22a–c). However, there is a tendency for hori- downdraft to the west seen in the upper part of Fig. 11, zontal vorticity to be more streamwise than crosswise as most prominently from 0443 to 0451 UTC. This is similar parcels initially rise. This same characteristic is present to the import of streamwise vorticity equatorward into for parcels located within the mesovortex at 0447 UTC supercell mesocyclones shown by Orf et al. (2017) and (Fig. 22d). From about 6 to 8 min prior to encountering Coffer and Parker (2017). 126 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 20. Median, 10th and 90th percentiles of streamwise (green) and crosswise (brown) horizontal vorticity and mean vertical vorticity tendencies (blue: tilting; red: stretching) for parcels from the environmental source region that are located in the mesovortices at (a) 0441, (b) 0443, (c) 0445, and (d) 0447 UTC. The parcel time series are shifted in time such that the time of maximum stretching tendency (i.e., as they encounter the low-level updraft) along each trajectory occurs at the same time (defined as t 2 0 in the figure). The heights of the parcels are normalized by their starting height (100–300 m AGL) prior to computing the mean parcel height shown by the black dashed line. This method helped to preserve signals that were susceptible to being lost by averaging over real time rather than a shifted time. The numbers of parcels in each set is given in the lower left of each panel.

Finally, the same analysis as above is shown for parcels developing mesovortex, these parcels descended rapidly originating in the RIJ source region (Fig. 23). Very few to 400–600 m AGL while attaining higher horizontal parcels located in the mesovortex at 0441 and 0443 UTC vorticity values. This descent is a reflection of a strong came from the RIJ, but for the sake of completeness, meso-g-scale downdraft that occurred within the rear- analyses for these groups of parcels are shown (Figs. 23a,b). inflow jet (depicted in the forecasts in Fig. 10 and in Parcels originating in the RIJ that were located in the observations in Fig. 15). Parcels at all times generally mesovortex at 0445 and 0447 UTC show a complex acquired more streamwise than crosswise horizontal evolution (Figs. 23c,d). Both sets of parcels originated vorticity as they descended (especially the parcels lo- much higher than parcels from the other two source cated in the mesovortex at 0447 UTC; Fig. 23d) prior to regions (;1000 vs 100–300 m AGL) and were charac- encountering the low-level mesovortex updraft. This is terized by lower values of horizontal vorticity (relative seen by the initially small horizontal vorticity (Figs. 19a,b) to the other source regions) that had similar magnitudes becoming larger and oriented in the direction of the of streamwise and crosswise components. Then, 6–7 min storm-relative wind (westerly) as the parcels descended prior to encountering the low-level updraft of the and approached the low-level mesovortex (Figs. 19c,d). JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 127 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 21. Parcel quantities for two representative environmental trajectories terminating in the mesovortices at 0447 UTC. (a),(c) Integrated and interpolated vertical vorticity as well as parcel height AGL, and (b),(d) the tilting, stretching, divergence, and baroclinic vertical vorticity tendency terms. The divergence term is nonzero (especially within the low-level updraft) because incompressibility is not assumed along the trajectories.

Also, although the RIJ parcels originated nearly 1 km processes similar to those described by Davies-Jones and AGL, on average, they entered the mesovortex while Brooks (1993) for supercell downdrafts. rising around 600 m AGL (Fig. 23). Although in- In all, multiple processes influenced the development of accuracies of the vertical vorticity tendency budgets vertical vorticity in the low-level mesovortex. Air parcels along these trajectories prevent a precise quantitative entering the mesovortex originated in both the environ- interpretation of how this streamwise vorticity is attained ment ahead of the QLCS and storm-cooled air behind the along the trajectories, the parcels reside in a strong hori- main gust front. Vorticity along these trajectories evolved zontal gradient in density potential temperature associ- differently depending on the source region of the parcels, ated with the strong rear-inflow surge/potentially cold as is further discussed and summarized below. downdraft (seen near the ground from 0445 to 0451 UTC in Fig. 11), which generates horizontal vorticity baro- 5. Summary and discussion clinically pointed to the east-southeast along the storm- relative flow. During descent, tilting of these parcels This study examined processes leading to the forma- would result in the generation of vertical vorticity (see tion of a tornadic mesovortex that occurred in south- increasing parcel trace thicknesses in Fig. 19d)via eastern South Dakota on 6 July 2015 during the PECAN 128 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 22. As in Fig. 20, but for the parcels from the northern source region. The vertical vorticity tendency terms are not included because the integrated vertical vorticity along the parcels was close to the interpolated vertical vorticity for only a few parcels in this airstream. The times are normalized in this case according to when the parcels reach their maximum vertical velocity in the lowest 2000 m, now shown by the red dashed line.

field experiment. This mesovortex and associated MCS which increased low-level speed and directional shear was well observed by multiple mobile Doppler radars, (as similarly observed by a lidar) and created an envi- , aircraft, and profiling systems. Mobile radar, ronment more conducive for the formation of meso- radiosonde, and aircraft data were assimilated into an en- vortices. Also as observed, the simulated mesovortex semble system to produce analyses and forecasts of the formed along the gust front of the main MCS near the event. The ensemble member that best captured important secondary gust front and to the northwest of ordinary details in the observed event, member 30, was selected for cells. Finally, the merger of two initial mesovortices into further analysis. Most of these analyses were performed the main mesovortex in the simulation may have also on a grid with 333-m horizontal resolution, which was occurred in reality, as observed by KFSD. sufficient to capture the meso-g-scale details of the Parcels entering the low-level mesovortex mainly mesovortex and processes influencing its development. originated from three different source regions: (i) the Forecasts captured several observed mesoscale fea- environment, (ii) the north of the mesovortex in storm- tures, including the eastward-moving gust front of the cooled air, and (iii) the northwest of the mesovortex in main MCS, a southward-moving gust front associated the vicinity of a localized downdraft/RIJ. Characteristics with a line of broken storms to the north, and several of parcels flowing toward the mesovortex from these ordinary cells that formed in the inflow region of the different regions are summarized in Fig. 24. Parcels main MCS. Outflow from these cells ahead of the main originating in the environment generally remained MCS locally enhanced the synoptic-scale southerly flow, close to the surface (i.e., 100–300 m AGL), were forced JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 129 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 23. As in Fig. 22, but for the parcels from the RIJ source region. upward along the southern portion of a westerly gust AGL with initially small horizontal vorticity that in- front surge, and then entered the low-level mesovortex creased during descent and became much more stream- near 1 km AGL. Horizontal vorticity in the environment wise just prior to the parcels reaching the mesovortex. was predominantly crosswise close to the surface and Parcels in the RIJ entered the mesovortex at lower alti- became even more crosswise as the parcels approached tudes (around 600 m AGL) than those from the other the mesovortex. This horizontal vorticity was generally source regions and also contained the most streamwise oriented toward the west as environmental parcels were vorticity prior to entering the low-level updraft. This forced up the gust front before entering the eastern edge streamwise horizontal vorticity was then likely converted of the mesovortex. The conversion of mostly crosswise into vertical vorticity within the low-level updraft via horizontal vorticity into vertical vorticity also likely tilting, producing a low-level, helical mesovortex. explains the presence of the simulated anticyclone to the In all, the mesovortex genesis process outlined in this west of the mesovortex, with line-perpendicular vortex study has characteristics that are similar to the genesis of lines (not shown) tilted upward in the mesovortex and low-level mesocyclones in supercells, similar to the downward in the cold pool. Parcels originating poleward cyclonic-only mesovortices examined in the case study of the mesovortex also began close to the surface and of Atkins and St. Laurent (2009b). They identified an gradually ascended as they flowed southward. Hori- RIJ source region of descending parcels (see Fig. 1 zontal vorticity associated with these parcels became herein and their Fig. 9), as well as parcels originating in more streamwise during their final approach to the the environment that import horizontal vorticity in- mesovortex, at which point they were tilted upward in a herent in the low-level shear. In both studies, parcels streamwise manner and entered the mesovortex near entering the low-level mesovortex from the environ- 1 km AGL. Finally, RIJ parcels originated near 1 km ment attained cyclonic vertical vorticity via tilting of 130 MONTHLY WEATHER REVIEW VOLUME 147 Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020

FIG. 24. Summary of the evolution of horizontal vorticity from the three airstreams identified in the development of the MV. Breaks in parcel trajectories indicate the height at which the parcel entered the mesovortices. horizontal vorticity and subsequent stretching of vertical updraft along the gust front long enough to generate vorticity. However, an important difference between the significant cyclonic vertical vorticity before entering the processes outlined by Atkins and St. Laurent (2009b) mesovortex updraft. and those here relates to how that cyclonic vertical Parcels originating to the northwest in the RIJ origi- vorticity develops from the environmental parcels. Since nated much higher than the parcels in the other two this process has not been identified in past literature to source regions—nearly 1 km AGL—but ended up closer the extent of our knowledge, a schematic summarizing to the surface as they entered the developing mesovortex. the evolution of vorticity along environmental parcels is These RIJ parcels contained the most streamwise vor- presented in Fig. 25.InAtkins and St. Laurent (2009b), ticity and would therefore more readily produce a low- the low-level mesovortex inflow parcels come from the level, upright, helical mesovortex via tilting of horizontal east and ingest largely streamwise horizontal vorticity vorticity. This process of attaining streamwise vorticity into the updraft directly from the environment (see during descent is consistent with the baroclinic genera- Fig. 1). In our analysis, the horizontal vorticity is ori- tion of streamwise vorticity along downdraft parcels ented at a much larger angle to the storm-relative wind, destined for supercell mesocyclones (e.g., Davies-Jones with the horizontal vorticity vectors pointing to the west and Brooks 1993), as similarly found by Atkins et al. in the strong southerly flow. This results in parcels from (2005) and Atkins and St. Laurent (2009b). the environment attaining cyclonic vertical vorticity From a forecasting standpoint, forecasters have been through upward tilting of crosswise horizontal vorticity encouraged to examine the component of low-level ver- as they are forced upward along the gust front rather tical wind shear normal to the convective line, with 2 than through large streamwise horizontal vorticity from magnitudes greater than 15 m s 1 over the lowest 3 km the environment as in Atkins and St. Laurent (2009b). AGL associated with greater potential for mesovortices The enhancement in upward forcing along this portion along a QLCS gust front (Schaumann and Przybylinski of the gust front provided by the enhanced convergence 2012). This result stems from numerous observational between the strong rear-inflow surge impinging on the analyses of mesovortex-producing QLCSs (Schaumann inflow contributes to the strong upward tilting of and Przybylinski 2012), as well as understanding gathered the crosswise vorticity prior to the parcels entering the from numerical modeling results showing how meso- mesovortex. Given the largely south–north trajectory of vortex development occurs within environments initial- the inflowing parcels, the parcels reside in the forced ized with vertical wind shear limited to the direction JANUARY 2019 F L O U R N O Y A N D C O N I G L I O 131

that is shown to contribute to the cyclonic vertical vor- ticity of the mesovortex from environmental parcels. Therefore, it is shown that this configuration of wind shear can still contribute to mesovortices stemming from the low-level shear through the tilting of crosswise hori- zontal vorticity in the south-southeasterly storm-relative flow, as the parcels reside in the zone of forced ascent for longer periods than they otherwise would with a more westerly trajectory seen in past studies of mesovortex

environmental inflow. Furthermore, the strong rear- Downloaded from http://journals.ametsoc.org/mwr/article-pdf/147/1/107/4381701/mwr-d-18-0221_1.pdf by NOAA Central Library user on 11 August 2020 inflow surge acts to temporarily enhance the conver- gence and forced uplift of these parcels despite the suboptimal shear configuration and relatively small line- normal vertical wind shear magnitude [in the sense of Rotunno et al. (1988)]. Because of the bowing section of the gust front, the parcels emerge on the poleward side of the bowing segment with cyclonic vertical vorticity FIG. 25. Summary of the conversion of the strong low-level shear from crosswise environmental horizontal vorticity into cyclonic near the developing mesovortex. Without the bowing vertical vorticity along the rear-inflow surge (purple arrow inside segment, parcels emerge well behind the gust front away the cold pool) that contributes to the strengthening MV (depicted from the convective updrafts along the leading edge of by the pink column). The red arrows depict the full 3D vorticity the cold pool. Indeed, a local surge in rear inflow along vector (v), and the blue arrow depicts the baroclinic generation of v the QLCS gust front is one of the ingredients associated horizontal vorticity along the gust front (› /›t)bc that helps to turn the full vorticity vector toward the south while being tilted by the with strong and potentially tornadic mesovortices by forced updraft along the gust front. An inflow parcel trajectory Schaumann and Przybylinski (2012). This serves as evi- from a starting height of 100 m AGL is depicted by the broken dence that QLCS mesovortices can still occur and be yellow line. The enhancement in vertical velocity from the rear- tornadic even with a low-level shear vector that is largely inflow surge converging with the inflow parcels is shown by the parallel to the gust front and relatively small line-normal green area. The black line depicts the strong sub-500-m AGL shear in the environment, and the pink arrow depicts the storm (MV) wind shear. motion. Not depicted is the anticyclonic vertical vorticity on the From a numerical weather prediction standpoint, west side of the enhanced vertical motion (green) that results from current limitations of mesovortex predictability are the tilting of the crosswise horizontal vorticity (see the dashed pink clear. Rapid strengthening and decay occurred in under contours in Fig. 14). 20 min, similar to observations of the event and well within what is typical for tornadic mesovortices (Trapp normal to the convective line (e.g., Trapp and Weisman et al. 2005). Thus, forecasting mesovortices at least re- 2003). The stronger forced updraft along substantial cold quires high-resolution spatial grids with frequent output pools that results from moderate to strong line-normal times (e.g., every 5 min or less) that are currently un- vertical wind shear (Rotunno et al. 1988) is thought to available in real-time operational settings. If such a tool promote enhanced stretching and parcels that are not was available, forecasters would then need to compare swept quickly over the gust front in the more erect up- model output with real-time observations, including gust drafts in these environments. However, in the present front locations and environmental boundaries. In all, the study, the low-level vertical wind shear is complex and ensemble forecasting system used here, which has since undergoes significant evolution as the MCS approaches. evolved into the NEWS-e (Wheatley et al. 2015; Jones A layer of line-normal shear (relative to the main MCS et al. 2016), showed skill in forecasting even storm-scale 2 cold pool) of 10–15 m s 1 exists from ;0.3 to 1.2 km AGL aspects of this mesovortex event and would be beneficial prior to modification (see Figs. 4 and 13a–d), but this line- to operational forecasters once computational capabil- normal wind shear becomes smaller with time as the ities allow for it. system approaches (Figs. 13e,f). This is especially true in the lowest ;500 m AGL with increasing southerly winds Acknowledgments. This study was supported by Na- with height in both observed and simulated hodographs tional Science Foundation Grant 1359726 and by a Na- (Figs. 13e,f). This very large vertical wind shear in the tional Science Foundation Graduate Student Fellowship lowest ;500 m AGL is oriented at a small angle to the obtained by the first author. The authors thank Conrad gust front, and it is the horizontal vorticity associated with Ziegler for helpful discussion of topics related to this line-parallel vertical wind shear below 500 m AGL this case. We also thank all participants of PECAN for 132 MONTHLY WEATHER REVIEW VOLUME 147 becoming temporarily nocturnal and for their dedica- propagation. Mon. Wea. Rev., 145, 3419–3440, https://doi.org/ tion in collecting the data used in this study. A NOAA/ 10.1175/MWR-D-16-0406.1. Office of Atmospheric Research (OAR)/Office of Weather Coffer, B. E., and M. D. Parker, 2017: Simulated supercells in non- tornadic and tornadic VORTEX2 environments. Mon. 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